Two holes should be also bored in the other flange to receive the two screws, which are to hold the magnet to the base, as shown at [Fig. 21] B. The magnet is now quite equal to the horse-shoe form, and must be made quite soft by annealing. This is done by heating it in a clear coal fire to a bright red heat, then burying it in hot ashes, and allowing it to cool gradually for a period of from 12 to 24 hours; or perhaps a better guide to the process will be to say, bury the iron in the hot ashes and leave it there until both it and they are quite cold. The iron must be brought to a bright cherry red heat before allowing it to cool, to soften it properly, and on no account must the cooling be hurried, or the metal will be hard. Iron is rendered hard by hammering, by being rapidly cooled, either in cold air or water,
and hard iron retains magnetism for a longer time than soft iron. As we wish to have a magnet that will only act as such when a current of electricity is passing around it, and shall return to the state of a simple piece of unmagnetised iron when the current is broken, we take the precaution of having it of soft iron. Many bells have failed to act properly, because this precaution has been neglected, the "residual" (or remaining) magnetism holding down the armature after contact has been broken. When the magnet has been annealed, its legs should be polished with a piece of emery cloth, and the ends filed up level and smooth. If it is intended to fasten the cores into the base-plate, this also should be annealed, unless it be made of brass, in which case a thin strip of soft iron should connect the back ends of the two legs before they are attached to the brass base (an iron yoke is preferable, as it certainly is conducive to better effects to have a massive iron yoke, than to have a mere strip as the connecting piece). It will also be readily understood and conceded that the cores should be cut longer when they are to be fastened by nuts, to allow a sufficient length for screwing the ends to receive the nuts. The length and size of the legs given above are suitable for a 2½ in. bell only;
for larger bells the size increases 1/16 of an inch, and the length ¼ of an inch, for every ½ in. increase in the diameter of the bell.
Fig. 20.
Fig. 20 A.